The formation of resist masks in the manufacture of integrated circuits using radiation sensitive layers which contain various organic polymers is known. Layers of the radiation sensitive material are formed on a substrate, exposed to light or other activating radiation such as electron beam or x-rays in a patternwise manner, and a visible image is developed by removing more soluble portions of the layer with a developing solution. In order to improve the characteristics of the remaining portions of the resist layer, such as its adhesion to various substrates, or its resistance to chemical and/or thermal degradation, the patterned resist layer is conventionally baked at an elevated temperature. The thermoplastic polymer materials in the resist are subject to flow at temperatures above their second order glass transition temperature. This tendency to flow distorts the resist image pattern, and in the fine geometry patterns which are required, for example, in integrated circuit manufacture the distortion can result in dimensional changes or even the closing of fine lines. Such distortion can also take place where the resist image is heated during the processing of the substrate such as by hot etchant solutions, ion implantation or plasma etching.
One solution to the problem has been the incorporation of free radical initiators into the resist structure such as is described in copending application Ser. No. 699,927 filed June 25, 1976 assigned a common assignee. While this produces resist masks having improved chemical and thermal stability, it does have the effect of lowering the sensitivity of the resist materials to radiation resulting in a need for longer exposure times.
U.S. Pat. No. 3,920,483 describes a resist hardening process used prior to ion implantation, in which the resist mask is subjected to an RF gas plasma oxidation to reduce the thickness of the photoresist layer and to limit resist flow during the ion implantation step.
U.S. Pat. No. 4,125,650, assigned to a common assignee, discloses a process for hardening resist masks, characterized by applying a coating of an agent comprising water soluble salts of orthoquinone diazide sulfonic and carboxylic acids. The resist mask coated with this agent is heated to cause the agent to form a hard crust or coating on the surface of said resist mask. However, since hardening of the resist is accomplished by a reaction of the coating agent with the resist components, the process is limited to resist systems that facilitate such reaction. The hardening process described herein can be used with any resist system that can be hardened by heating.
In addition, the process described in the above reference forms a hard continuous layer over the resist surface which prevents solvent evaporation from the resist during baking. This results in explosions, during baking, in large non-patterned areas of resist (larger than 100.times.100.mu.).
An improved process has now been found for hardening resist images. It prevents image flow due to resist heating and maintains not only the image size in the resist but also the resist profile shape after prolonged heating at temperatures of about 230.degree. C. or higher.